Quite large areas of land devoted to the cultivation of winter crops are covered with tunnel-like structures, low tunnels, walk-in tunnels and greenhouses; using polyethylene sheeting for protection of the crops from the cold. The polymeric substances are chosen in such manner as to permit light in the visible range to penetrate said sheeting. When the visible light penetrates the sheeting and reaches the ground or the growing plant, part of it is converted into light of longer wavelength, in the infra-red range, and generally the temperature inside such tunnels is higher than the outside temperature. At night, when no illumination by sun-light takes place, heat will be lost through the polyethylene sheeting through infra-red radiation passing through said sheeting, as this is generally quite pervious to such radiation. The resulting drop of temperature is harmful to the growth of the plants and hitherto various attempts have been made to provide compositions of matter which will substantially prevent the heat loss due to infra-red radiation.